Treatment with elevated temperatures (42 degrees - 43 degrees) causes regression of tumours and it is planned to investigate the possible role of activation of lysosomal enzymes and damage to lysosomal membrane in the regression. The effect of temperature will be compared with, and combined with that of irradiation on lysosomes of normal and tumour tissues studied by quantitative histochemical methods which we have developed recently. Lysosomal labilisers will be investigated and it is considered possible that the investigation may demonstrate some important differences between the structure and permability of membranes of normal tissue and tumour cells. This study of the role of the lysosomes in tumour regression caused by hyperthermia and irradition may lead to more effective treatment of tumours in man by radiotherapy.